Aluminum is produced in Hall-Heroult cells by the electrolysis of alumina in molten cryolite, using conductive carbon electrodes as anodes. During the reaction the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction ##STR1##
The problems caused by the consumption of anode carbon are related to the cost of the anode consumed in the above reaction and to the impurities introduced into the melt from the carbon source. The petroleum cokes used in manufacturing the anodes usually have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing particularly troublesome workplace and environmental pollution. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon were consumed in the reduction the overall reaction would be 2Al.sub.2 O.sub.3 .fwdarw.4Al+3O.sub.2 and the oxygen produced could theoretically be recovered, but more importantly no carbon would be consumed at the anode and no contamination of the atmosphere or the product would occur from the impurities present in the coke.
Attempts have been made previously to use non-consumable anodes in aluminum reduction cells with little apparent success. Metals either melt at the temperature of cell operation, or are attacked by oxygen or by the cryolite bath. Ceramic compounds such as oxides with perovskite and spinel crystal structures usually have too high electrical resistance or are attacked by the cryolite bath.
One of the problems arising in the development of conductive ceramic anodes has been caused by the difficulty of making a durable electrical connection between the anode and the lead-in current conductor. Previous efforts in the field have produced connectors, constructed primarily of metals such as silver, copper, and stainless steel. Can, U.S. Pat. No. 3,681,506, discloses a resilient metal washer held in place to form an electrical connection. Davies, U.S. Pat. No. 3,893,821, discloses a contact material containing Ag, La, SrCrO.sub.3 and CdO. Douglas et al., U.S. Pat. No. 3,922,236, disclose a contact material containing Ag, Cu, La, and SrCrO.sub.3. Fletcher, U.S. Pat. No. 3,990,860, discloses cermet compositions containing stainless steel or Mo in a matrix of Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3. Shida et al., U.S. Pat. No. 4,141,727, disclose contacts of Ag, Bi.sub.2 O.sub.3, SnO.sub.2 and Sn. Schirnig et al., U.S. Pat. No. 4,247,381, disclose an electrode useful for AlCl.sub.3 electrolysis comprising a graphite pipe, a metallic conductor with a melting point below the bath temperature, and a protective ceramic pipe surrounding the former. West German Pat. No. 1,244,343, U.S. Ser. No. 729,621, discloses borides or carbides of Ti, Zr, Ta, or Nb cast to Al using a flux of Li.sub.3 AlF.sub.6, Na.sub.3 AlF.sub.6 and NaCl. Alder, U.S. Pat. No. 4,357,226, discloses an anode assembly for a Hall cell comprising individual units mechanically held together by a clamping arrangement. U.K. patent application No. 2,078,259 published Jan. 6, 1982 (and equivalent U.S. Pat. No. 4,397,729 issued Aug. 9, 1983) describes the use of mixed oxides, alloys, composites and cermets including ferrites or chromites for use as inert anode materials. U.S. Pat. No. 4,374,761 issued Feb. 22, 1983 to S. P. Ray describes an inert electrode for electrolytic production of metals dissolved in molten salt comprising cermets containing metal powders including Ni and Cu. Our application, Ser. No. 475,951, filed Mar. 16, 1983, U.S. Pat. No. 4,443,314, discloses a cermet anode connector.
In non-consumable anodes, ceramics such as stannic oxide, ferrites, spinels, perovskites and various cermets are principal materials under study. A cermet is a composite material containing both metal and ceramic phases.